1. Field of the Invention
The present invention relates to a high side switch circuit, an interface circuit including the high side switch circuit, and an electronic device including the interface circuit.
2. Description of the Background Art
A switch connected between a power supply and a load is generally referred to as a high side switch. If a current flowing through the high side switch becomes excessively high because of line short-circuit or failure of load, the high side switch would be damaged. In view of the foregoing, high side switch circuits including a high side switch and a protection circuit for protecting the high side switch from over-current have been proposed.
By way of example, Japanese Utility Model Laying-Open No. 7-11031 discloses a semiconductor high side switch utilizing an MOSFET (Metal Oxide Semiconductor Field Effect Transistor) as a high side switch, and an over-current protection sensor for the high side switch. Specifically, the over-current protection sensor includes a reference voltage generating circuit connected to one terminal of the MOSFET for generating a reference voltage and a comparator connected to an output terminal of the reference voltage generating circuit and the other terminal of the MOSFET. The comparator compares the on voltage of MOSFET with the reference voltage, to detect an over-current state of the MOSFET.
According to Japanese Utility Model Laying-Open No. 7-11031, the reference voltage generating circuit generates the reference voltage by dividing power supply voltage using a plurality of series-connected resistive elements. Accuracy of the reference voltage depends on the accuracy of resistance values of the resistive elements.
Japanese Utility Model Laying-Open No. 7-11031 describes that the semiconductor high side switch and the over-current protection sensor circuit are integrated into one IC (Integrated Circuit). Here, the resistive elements are formed on a semiconductor chip. Generally, however, it is difficult to attain high accuracy of resistance value of the resistive elements formed on a semiconductor chip.
As a method of improving accuracy of resistance value of a resistance circuit included in a semiconductor integrated circuit, adjustment of resistance value by fuse resistor has been known. Typically, a fuse resistor is blown by causing a large current to flow through the fuse resistor. This adjusts the resistance value of the resistance circuit. Considering the possibility of fuse resistor blowing, however, it is difficult to place a fuse resistor in a path through which a relatively large current flows.
Japanese Utility Model Laying-Open No. 7-11031 further describes detection of an over-current state of the MOSFET. Japanese Utility Model Laying-Open No. 7-11031, however, does not specifically describe protection of the semiconductor high side switch when an over-current is detected. Further, if an over-current flows through the semiconductor high side switch, protection of circuitry including the load and power supply connected to the high side switch must also be considered. Japanese Utility Model Laying-Open No. 7-11031, however, does not disclose any specific configuration to solve such a problem.
An object of the present invention is to provide a high side switch circuit enabling improved accuracy of detecting over-current in the high side switch, as well as a device including the high side switch circuit.
Another object of the present invention is to provide a high side switch circuit capable of protecting not only the high side switch but also circuitry connected to the high side switch if an over-current in the high side switch is detected, as well as to provide a device including the high side switch circuit.